The present invention relates to a coded data control device and more particularly to a coded data control device for use in a digital video recorder-player which uses a read-only recording medium such as a CD-ROM and a rewritable recording medium such as an optical magnetic disk.
Disc-recording media such as a magnetic disc, optical disc, optical magnetic disc and so on allow high-speed random accessing any location of data recorded therein. The use of the quick random access recording medium makes it possible to reproduce data recorded in different areas thereof as a sequence of continuously recorded data. Video data has a very high data rate and can not easily be recorded into the recording medium without previously being processed. However, by high efficiency encoding video and audio signals it is possible to reduce the data rate of them to a degree whereat the data may be recorded in the disc medium without decreasing quality of video and audio data.
An example of a known high-efficiency coding method is a MPEG (Moving Picture Experts Group) method which has been promoted to be standardized by ISO-IEC/JTC1/SC29/WG11 as a method of high-efficiency encoding video and audio signals and multiplexing for synchronous reproduction of the signals. Video coding by the MPEG method uses a group of pictures (GOP) for example consisting of 12 or 15 video frames as a unit to be coded by prediction.
Video frames in a GOP are classified into I(Intra picture)-frames, P(Predictive picture)-frames and B(Bidirectional picture)-frames. An I-frame is to be intraframe-coded. A P-frame is to be coded by forward predicting from the I-frame or by forward predicting from another P-frame. A B-frame is located between an I-frame and a P-frame or between P-frames and encoded by bi-directional predicting from an I-frame and a P-frame or from a precedent and subsequent P-frames.
To reproduce the GOP, it is necessary to decode an I-frame first since P-frames and B-frames are encoded by prediction from an I-frame or from a video-frame coded by prediction from an I-frame. Otherwise, all other frames can not be decoded. Namely, in case of reproducing video from a midway of it in sequence, it is needed to start decoding an I-frame. P-frames can be decoded easier than B-frames since they are coded by prediction from a preceding frame only.
I-frames and P-frames are hereinafter referred to as key-frames. A video sequence denotes a series of successive video-frames recorded for an interval between the moment recording starts and the moment recording ends and a series of corresponding coded data. A quantity of codes obtained by the MPEG coding method is substantially constant in average but is out of proportion to time in a short period. Consequently, spacing between coded video-data in each frame is not constant and recording positions of coded data in each frame can not be uniquely determined. It is, therefore, difficult to reproduce discrete video-frames continuously, like fast playback.
In Japanese laid-open patent publication No. 5-153577, there is disclosed an example of conventional art for realizing fast playback of high-efficiency coded data. This method reads data from a disc medium at an increased speed, selects necessary data from the successively read-out data and reproduces them at a high speed.
As mentioned above, the prior art method reads out an increased quantity of data by increasing a reading speed but not using random accessing and selects therefrom data necessary for fast playback. The playback speed may increase as the data reading-out speed increases. However, increasing the data reading-out speed is limited to an extent and, therefore, the playback speed can not be increased enough.
To realize higher speed playback, it may be considered to read a disc recording medium by skipping unnecessary parts by using a random accessing technique. Namely, by repeating a cycle of data reading with subsequent quick jumping it is possible to change a speed of fast playback by each jumping value. However, in case of reading high-efficiency coded data, there may arise such a problem that when a jump is made to start reproduction from a midway of a video sequence, an I-frame shall be first decoded as aforementioned but a start point, i.e., coded video-data recorded therein can not be found because a quantity of codes is out of proportion to time.
The above-mentioned prior art method can not access data at random in a specified period and, therefore, can not realize features such as index searching and pointer edition using a random access. The index searching feature is to synchronously reproduce video output and audio output from a specified moment by using the random accessing function. The prior art requires much time for reading unnecessary data before finding multiplexed coded-data necessary for reproduction because location of an I-frame is unknown.
As aforementioned, high-speed playback of a disc medium is realized by reproducing only key frames, omitting unnecessary video frames. This requires repeating an operation cycle consisting of reading coded video-data and subsequent quick random accessing a next necessary data. The prior art can not find location of records of coded video-data of the key frames and, therefore, has to read a wide range of multiplexed coded-data including the key frames, resulting in decreasing the number of video-frames to be reproduced in a unit time. The pointer editing is used to designate a plurality of starting points and ending points and links a plurality of time-discrete data by using logical pointers without copying them. It assures synchronous reproduction of video output and audio output. In this case, it is also necessary to repeat a random access from one specified area to another area in the disc recording medium for reproduction. To get successively reproduced data, it is necessary to force a decoder to output reproduced data for a period no data is obtained. For this purpose, a code buffer must be provided for supplying coded data to be outputted by the decoder for an idling period.
The prior art may read unnecessary data at every random access, causing need for supplying buffer coded data for every period no coded-data to be decoded is obtained. The data readable from the disc medium is multiplexed coded-data having an added thereto header that must be read first when synchronously reproducing video and audio outputs.
A multiplexed coded-data is divided into so called packs, each of which has a first header placed at the head thereof. Each pack is composed of coded video-data with a second header and coded audio-data with a second header. The first header contains an information for synchronizing coded video-data with coded audio-data. The second header contains information indicating kinds of the data following thereto. Therefore, the first header must be first read out to realize synchronous reproduction of video and audio.